An all-solid-state battery is such a battery that the cathode, the anode and the electrolyte layer present between the electrodes are all solid. Among all-solid-state batteries, an oxide all-solid-state battery comprising an oxide solid electrolyte is advantageous in that it is easy to handle in the air compared to a sulfide all-solid-state battery that may produce hydrogen sulfide.
As an oxide solid electrolyte with excellent lithium ion conductivity, a Li—La—Zr—O-based solid electrolyte (i.e., a garnet-type oxide solid electrolyte) is known.
A carbon anode has been used as the anode of an all-solid-state battery. However, the theoretical capacity of the carbon anode is only 372 mAhg−1. Therefore, to increase the anode capacity, various kinds of anode materials have been developed. Of them, a silicon anode has attracted attention since, when it is sufficiently lithiated (Li22Si5), the theoretical capacity is as high as 4200 mAhg−1.
For example, a perovskite-type oxide solid electrolyte such as (Li, La)TiO3 and a nasicon-type oxide solid electrolyte such as Li(Al, Ti)(PO4)3 are known as oxide solid electrolytes. They contain elements such as Ti and Ga. The reduction potentials of Ti and Ga are higher than the potential of the silicon anode. Therefore, in oxide all-solid-state batteries comprising such oxide solid electrolytes, the silicon anode cannot be used since the oxide solid electrolytes cause reductive decomposition.
Meanwhile, since the reduction potential of the garnet-type oxide solid electrolyte is lower than the potential of the silicon anode, the garnet-type oxide solid electrolyte can be used in an oxide all-solid-state battery in combination with the silicon anode.
Patent Literature 1 discloses a lithium battery comprising a solid electrolyte layer which comprises Li, La, Zr, Al, Si and O, which has a garnet-type structure, and which is a sintered body. Also in Patent Literature 1, there is a description of the use of Si as an anode active material.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-018792
In Patent Literature 1, however, an experimental example of producing a battery by combining a garnet-type oxide solid electrolyte with an anode active material containing Si, is not disclosed. This is considered to be because the production or charge and discharge of the battery is difficult in the prior art due to the following reasons.
First, silicon particles are a very hard substance. Therefore, it is difficult to dispose such silicon particles and a solid electrolyte with securing a sufficient contact area therebetween.
Also, the garnet-type oxide solid electrolyte generally has a high sintering temperature of 1,200° C. Meanwhile, silicon itself has an oxidation temperature of about 700° C. When silicon is oxidized, it is converted into silicon oxide (SiO2). A battery containing such SiO2 cannot be charged and discharged. Therefore, when the garnet-type oxide solid electrolyte and silicon are tried to be joined by sintering them, SiO2 is produced before they are joined. Therefore, a joined body thus obtained cannot be used as a battery material.
Due to the above reasons, there is no known example of producing an all-solid-state battery by combining the silicon anode and the garnet-type oxide solid electrolyte.